The magnetic circuits in electric motors usually comprise a laminated core, e.g. of sheet steel with a welded construction. To provide ventilation and cooling the core is often divided into stacks with radial and/or axial ventilation ducts. For larger motors the laminations are punched out in segments which are attached to the frame of the machine, the laminated core being held together by pressure fingers and pressure rings. The winding is disposed in slots in the laminated core, the slots generally having a cross section in the shape of a rectangle or trapezium.
In multi-phase electric motors the windings are made as either single or double layer windings. With single layer windings there is only one coil side per slot, whereas with double layer windings there are two coil sides per slot. By coil side is meant one or more conductors combined vertically or horizontally and provided with a common coil insulation, i.e. an insulation designed to withstand the rated voltage of the motor to earth.
Double-layer windings are generally made as diamond windings whereas single layer windings in the present context can be made as diamond or flat windings. Only one (possibly two) coil width exists in diamond windings whereas flat windings are made as concentric windings, i.e. with widely varying coil width. By coil width is meant the distance in arc dimension between two coil sides pertaining to the same coil.
Normally all large motors are made with double-layer winding and coils of the same size. Each coil is placed with one side in one layer and the other side in the other layer. This means that all coils cross each other in the coil end. If there are more than two layers these crossings complicate the winding work and the coil end is less satisfactory.
It is considered that coils for rotating electric motors can be manufactured with good results up to a voltage range of 10-20 kV.
Large alternating current motors are divided into synchronous and asynchronous motors, the former generally covering a higher power range up to a few tens of MW and being constructed to be supplied with a voltage of normally maximally 20 kV. The synchronous motor operates with a rotor speed that is synchronous with the network frequency. In an asynchronous motor the magnetic field rotates faster than the rotor so that the induced currents will provide torque in the direction of rotation. The two types of motors are to a great extent similar in construction. They consist of a stator with a rotor placed inside the stator. The stator is built up of a laminated core with slots punched out for the winding. The stator is placed in a bottom box attached to the foundation by its feet. The rotor is suspended in bearings mounted on the box. A stator shell is placed on the bottom box to protect the active parts. The shell is provided with openings for cooling air to enter.
The function of an alternating current motor is based on interaction between magnetic fields, electric currents and mechanical motion. The magnetic fields are localized primarily in the iron of the machine and the electric currents are localized in the windings.
A distinction is made between two main types of alternating current motors: synchronous and asynchronous machines. The principal difference between synchronous and asynchronous machines is how the torque is produced. A synchronous motor is excited by supplying energy to the rotor from the outside via brushless exciters or slip rings, whereas an asynchronous motor obtains its excitation energy from the stator current through induction. The speed of the synchronous motor is therefore not as dependent on load as in the asynchronous motor.
Depending on the construction of the rotor, there are two types of synchronous motors: those with salient poles and those with a cylindrical rotor. In high-speed 2-pole operation the mechanical stresses on the rotor will be extremely high and in that case it is favourable to use a cylindrical rotor. For motors with lower speeds, four-pole or more, the rotor diameter will be larger. In view of the lower speed and thus correspondingly lower mechanical stresses, it is more favourable for the rotor to have salient poles.
The boundary between the two types is indefinite. At higher power and with four poles, cylindrical rotors are used that are long and slim in shape. At lower power and with four poles, rotors with salient poles are used.
Asynchronous motors are also divided into two types: squirrel-cage induction motors or slip ring motors. Common to both types is that the rotor is built up of laminations with slots for the rotor winding. The difference is in the construction of the winding. The squirrel-cage induction motors have a squirrel-cage winding consisting of axial rods that are short-circuited at the ends with a short-circuiting ring. Asynchronous motors with slip rings have a three-phase winding in the rotor with phase terminals connected to the slip rings.
By designing the rotor slots in various ways the start and operating properties of the squirrel-cage induction motor can be adjusted to various operating requirements. Slip-ring asynchronous motors are primarily used under difficult starting conditions. External resistance can be connected via the slip rings. By increasing the rotor resistance the maximum torque can be moved towards lower speed, thus increasing the start torque. When starting is complete the external start resistance is short-circuited.
The choice of a large alternating current motor as regards to type, nesting class and cooling method, is dependent on the following factors, among others:                Torque characteristic of the load        Type of load and load cycle        Start power restrictions        Network characteristics        Cost of electric energy        Environment where the motor is to be installed        Investment cost in relation to the estimated service life of the plant        
The main desire for an electric machine is that its capital cost and running costs shall be as low as possible. It is therefore desirable to keep the efficiency as high as possible at given power factors. The synchronous motor generally has higher efficiency than the asynchronous motor.
The rotor of a synchronous motor is often manufactured with salient poles. Its main use is in the power range of 1 MW to a few tens of MW, e.g. for grinding mills and refiners in the paper industry, for large pumps both in the process industry and in connection with weak networks, e.g. for irrigation installations in desert countries. The oil industry also uses large synchronous motors for pumps and compressors.
The main reason for using synchronous motors instead of the less expensive asynchronous motors is that the synchronous motor produces less stress on the network, in the form of lower start current, and that at over-excitation the synchronous motor can also be used to improve the power factor. Large synchronous motors may also have slightly higher efficiency than equivalent asynchronous motors.
The winding must be insulated, both between the winding turns in the coil and also between coil and surroundings. Various forms of plastic, varnish and glassfibre material are often used as insulating material. The coil ends are braced in order to counteract the forces appearing between the various coils, particularly at short-circuiting.
Motors of the type described above are connected to high-voltage networks of e.g. 145 kV through the use of a transformer which lowers the voltage. The use of a motor in this way, connected to the high-voltage network via a transformer entails a number of drawbacks. Among others the following drawbacks may be mentioned.                the transformer is expensive, increases transport costs and requires space        the transformer lowers the efficiency of the system        the transformer consumes reactive power        a conventional transformer contains oil, with the associated risks        involves sensitive operation since the motor, via the transformer, works against a weaker network        